Remember to Yield
Because a rod bolt or other engine fastener must give and take to a degree, it acts much like a spring when correctly selected and installed. Just as a spring can be overstretched, a rod bolt can permanently deform if installed or tightened incorrectly. The point at which a bolt will overstretch and deform is called the yield strength. Most performance-bolt manufacturers, like ARP, provide torque and stretch values equal to 75 percent of the fastener's yield strength. So when a particular bolt is tightened to spec, it still has another 25 percent of stretch to its yield strength. This allows the fastener to rebound and maintain a clamping load. For a rod bolt, the 25 percent remaining yield strength allows it to withstand added loads at high rpm. If the fastener length exceeds the yield strength, it is ruined and must be replaced.
Before tightening any new fastener (where the stretch method cannot be used), it's important to tighten and loosen five times. This is because the existing friction is at its highest value when a new fastener is first tightened. Each successive time the fastener is torqued and loosened, the value gets smaller, until finally the friction levels out and becomes constant. The number of cycles required is dependent upon the lubricant used. Generally, ARP lubricants require five cycles before tightening.
Get Clean & Greasy
Properly cleaning and lubing a rod bolt before tightening is also very important. As an example, a dry, nonlubed bolt may require 95 ft-lb of the applied torque to overcome friction during tightening. But before you just spray your favorite engine oil on your rod bolts, check with the bolt's manufacturer. Studies have shown that typical engine oil versus a bolt manufacturer's lubricant will produce a difference in the amount of torque needed to meet the desired stretch, because the friction coefficients of the lubricants vary considerably. Most bolt and stud manufacturers offer lubricants that have been tested to provide the recommended tightening values.
As a fastener is torqued in place, it has to overcome the friction between the threads and the component being secured. The remaining torque is what applies the tension (or stretch) that keeps the fastener from loosening (clamp load).
Using a Torque Wrench
For many applications, such as cylinder head bolts, nuts, and main cap studs or bolts, torque wrenches work fine. But a torque wrench only measures the friction encountered, not the stretch of the fastener. This can become a problem because the results vary and are hard to control. By using the stretch method where possible (as with rod bolts), the preload is controlled and independent of friction.
Bolts or Studs?
Should you use bolts or studs? It depends on the application, but for many high-performance engines, studs provide more accurate and consistent torque loading. This is because when bolts are used to hold an engine component, the fastener is actually being twisted as it's being torqued to the proper value. Therefore, the bolt is reacting to two forces at the same time.
When installing studs, remember that they should be installed hand-tight and never forced in tightly with a tool or jam nuts. This way, as torque is applied to the nut (to clamp down the component), the stud will only stretch on the vertical axis. Another benefit of studs is that there is less force applied to the block threads, which adds to block life. CHP